The present invention relates generally to the mixing or pumping of fluids or the like and, more particularly, to a number of systems, related components, and related methods for pumping or mixing fluids using a rotating magnetic bearing levitated by a superconducting element.
Most pharmaceutical solutions and suspensions manufactured on an industrial scale require highly controlled, thorough mixing to achieve a satisfactory yield and to ensure a uniform distribution of ingredients in the final product. Agitator tanks are frequently used to complete the mixing process, but a better degree of mixing is normally achieved by using a mechanical stirrer or impeller (e.g., a set of mixing blades attached to a metal rod). Typically, the mechanical stirrer or impeller is simply lowered into the fluid through an opening in the top of the vessel and rotated by an external motor to create the desired mixing action.
One significant limitation or shortcoming of such an arrangement is the danger of contamination or leakage during mixing. The rod carrying the mixing blades or impeller is typically introduced into the vessel through a dynamic seal or bearing. This opening provides an opportunity for bacteria or other contaminants to enter, which of course can lead to the degradation of the product. A corresponding danger of environmental contamination exists in applications involving hazardous or toxic fluids, or suspensions of pathogenic organisms, since dynamic seals or bearings are prone to leakage. Cleanup and sterilization are also made difficult by the dynamic bearings or seals, since these structures typically include folds and crevices that are difficult to reach. Since these problems are faced by all manufacturers of sterile solutions, pharmaceuticals, or the like, the U.S. Food and Drug Administration (FDA) has consequently promulgated strict processing requirements for such fluids, and especially those slated for intravenous use.
Recently, there has also been an extraordinary increase in the use of biosynthetic pathways in the production of pharmaceutical materials, but problems plague those involved in this rapidly advancing industry. The primary problem is that suspensions of genetically altered bacterial cells frequently used to produce protein pharmaceuticals (insulin is a well-known example) require gentle mixing to circulate nutrients. If overly vigorous mixing or contact between the impeller and the vessel wall occurs, the resultant forces and shear stresses may damage or destroy a significant fraction of the cells, as well as protein molecules that are sensitive to shear stresses. This not only reduces the beneficial yield of the process, but also creates deleterious debris in the fluid suspension that requires further processing to remove.
In an effort to overcome this problem, others have proposed alternative mixing technologies. The most common proposal for stirring fluids under sterile conditions is to use a rotating, permanent magnet bar covered by an inert layer of TEFLON, glass, or the like. The magnetic bar is placed on the bottom of the agitator vessel and rotated by a driving magnet positioned external to the vessel. Of course, the use of such an externally driven magnetic bar avoids the need for a dynamic bearing, seal or other opening in the vessel to transfer the rotational force from the driving magnet to the stirring magnet. Therefore, a completely enclosed system is provided. This of course prevents leakage and the potential for contamination created by hazardous materials (e.g., cytotoxic agents, solvents with low flash points, blood products, etc.), eases clean up, and allows for the desirable sterile interior environment to be maintained.
However, several well-recognized drawbacks are associated with this mixing technology, making it unacceptable for use in many applications. For example, the driving magnet produces not only torque on the stirring magnetic bar, but also an attractive axial thrust force tending to drive the bar into contact with the bottom wall of the vessel. This of course generates substantial friction at the interface between the bar and the bottom wall of the vessel. This uncontrolled friction generates unwanted heat and may also introduce an undesirable shear stress in the fluid. Consequently, fragile biological molecules, such as proteins and living cells that are highly sensitive to temperature and shear stress, are easily damaged during the mixing process, and the resultant debris may contaminate the product. Moreover, the magnetic bar stirrer may not generate the level of circulation provided by an impeller, and thus cannot be scaled up to provide effective mixing throughout the entire volume of large agitation tanks of the type preferred in commercial production operations.
In yet another effort to eliminate the need for dynamic bearings or shaft seals, some have proposed mixing vessels having external magnets that remotely couple the mixing impeller to a motor located externally to the vessel. A typical magnetic coupler comprises a drive magnet attached to the motor and a stirring magnet carrying an impeller. Similar to the magnetic bar technology described above, the driver and stirrer magnets are kept in close proximity to ensure that the coupling between the two is strong enough to provide sufficient torque. An example of one such proposal is found in U.S. Pat. No. 5,470,152 to Rains.
As described above, the high torque generated can drive the impeller into the walls of the vessel creating significant friction. By strategically positioning roller bearings inside the vessel, the effects of friction between the impeller and the vessel wall can be substantially reduced. Of course, high stresses at the interfaces between the ball bearings and the vessel wall or impeller result in a grinding of the mixing proteins and living cells, and loss of yield. Further, the bearings may be sensitive to corrosive reactions with water-based solutions and other media and will eventually deteriorate, resulting in frictional losses that slow the impeller, reduce the mixing action, and eventually also lead to undesirable contamination of the product. Bearings also add to the cleanup problems.
In an effort to address and overcome the limitations described above, still others have proposed levitated bearings designed to reduce the deleterious effects of friction resulting from magnetically coupled mixers. By using a specially configured magnetic coupler to maintain only a repulsive levitation force in the vertical direction, the large thrust force between the stirring and driving magnets can be eliminated, along with the resultant shear stress and frictional heating. An example of one such arrangement is shown in U.S. Pat. No. 5,478,149 to Quigg.
However, one limitation remaining from this approach is that only magnet-magnet interactions provide the levitation. This leads to intrinsically unstable systems that produce the desired levitation in the vertical direction, but are unable to control side-to-side movement. As a result, external contact bearings in the form of bearing rings are necessary to laterally stabilize the impeller. Although this xe2x80x9cpartialxe2x80x9d levitation reduces the friction between the impeller and the vessel walls, it does not totally eliminate the drawbacks of the magnetically coupled, roller bearing mixers previously mentioned.
In an effort to eliminate the need for contact or other types of mechanical roller bearings, complex feedback control has been proposed to stabilize the impeller. Typical arrangements use electromagnets positioned alongside the levitating magnet. However, the high power level required to attain only sub-millimeter separations between the levitating magnet and the stabilizing magnets constitutes a major disadvantage of this approach. Furthermore, this solution is quite complex, since the stabilizing magnets must be actively monitored and precisely controlled by complex computer-implemented software routines to achieve even a moderate degree of stability. As a consequence of this complexity and the associated maintenance expense, this ostensible solution has not been accepted in the commercial arena, and it is doubtful that it can be successfully scaled up for use in mixing industrial or commercial scale process volumes.
Still others have proposed the use of superconductive materials to levitate magnetic bearings. Despite recent advances in the art, significant limitation on the application of this technology to mixing systems results from the extraordinarily cold temperatures required to create the desired superconductive effects. Even the recently discovered xe2x80x9chigh temperaturexe2x80x9d superconductors require temperatures on the order of 77 to 130 Kelvin to induce reliable, stable levitation in a magnetic bearing. In the past, the relatively wide separation distance required between the bearing, the cryostat outer wall, and the superconducting element necessary to prevent unwanted cooling of the fluid has limited the industrial applicability of this approach. To date, applications of this technology to fluids have been primarily in the pumping of cryogens or the like, such as those typically used in cold fusion experiments, in flywheels or other energy storage devices, or for space travel (see representative U.S. Pat. No. 5,747,426 to Abboud or U.S. Pat. No. 4,365,942 to Schmidt), where there is of course little concern for the inevitable cooling effect created.
In my prior U.S. Pat. No. 5,567,672, I describe levitating a magnet above a superconducting element in a cryostat, which contains the cooling source used to cool the superconducting element. This arrangement could possibly be used as part of a system for mixing temperature sensitive fluids, such as cell suspensions or blood, as disclosed herein. However, the resultant increased separation created by the double wall vacuum gap may decrease the stability and the load capacity of the levitating magnet. This may limit the applications in which this arrangement is useful, and could especially preclude use with particularly viscous fluids or with the large volumes of fluid typically present in commercial scale operations.
Thus, a need is identified for an improved system having a levitating magnetic bearing for mixing or pumping fluids, and especially ultra-pure, hazardous, or delicate fluid solutions or suspensions. The system would preferably employ a magnetic bearing that levitates in a stable fashion to avoid contact with the bottom or side walls of the vessel. Since the bearing would levitate in the fluid, no mixing rod or other structure penetrating the mixing vessel would be necessary, thus eliminating the need for dynamic bearings or seals and all potentially deleterious effects associated therewith. Since penetration is unnecessary, the vessel could be completely sealed prior to mixing to avoid the potential for contamination and reduce the chance for exposure in the case of hazardous or biological fluids, such as contaminated blood or the like. The vessel and magnetic bearing could also be made of disposable materials and discarded after each use, which would eliminate the need for cleaning or sterilization. The absence of a mixing or stirring rod penetrating through the vessel would also allow a slowly rotating impeller to be held at an off-axis position in a sealed vessel, thus making it possible to independently rotate the vessel about its central axis to achieve very gentle, yet thorough, mixing.
In the case of warm or temperature-sensitive fluids, the use of superconductivity to provide the desired levitation would be possible by thermally isolating and separating the superconducting element from the magnetic bearing and providing a separate, substantially isolated cooling source. This combined thermal isolation and separation would avoid creating any significant cooling in the vessel, the magnetic bearing or the fluid being mixed or pumped. The use of a superconductor would also eliminate the sole reliance on magnet-magnet repulsion to provide the levitation force and the concomitant need for active electronic control systems to ensure stable levitation. Overall, the proposed system would have superior characteristics over existing mixing or pumping technologies, especially in terms of sterility, mixing quality, safety and reliability, and would be readily adaptable for use in larger, industrial scale operations.
To meet these needs, and in accordance with a first aspect of the present invention as described herein, a number of systems that are capable of pumping or mixing fluids, including temperature sensitive fluids, using a magnetic bearing, impeller, rotor or other element or device capable of generating a pumping or mixing action in a fluid (hereinafter generically referred to as a xe2x80x9cmagnetic bearingxe2x80x9d) levitated by a superconducting element are disclosed. The magnetic bearing may be placed in a vessel positioned adjacent to the wall of a cryostat or other housing for the superconducting element. A separate cooling source thermally linked to the superconducting element provides the necessary cooling to create the desired superconductive effects and induce levitation in the magnetic bearing. The cryostat outer wall or other housing may define a chamber around the superconducting element. This chamber thermally isolates the superconducting element from the vessel containing the bearing. To minimize thermal transfer from the superconducting element to the outer wall or housing, this chamber is preferably evacuated, but may be instead filled with an insulating material. This thermal isolation and separation means that the superconducting element may be placed in close proximity to the outer wall of the cryostat or other housing adjacent to the vessel to achieve a significant reduction in the separation distance between the levitating bearing and the superconducting element. This advantageously enhances the magnetic stiffness and loading capacity of the bearing as it levitates. However, since the superconducting element may be thermally isolated from the wall or housing, the magnetic bearing, and hence the vessel and fluid contained therein, are not exposed to the cold temperatures required to generate the desired superconductive effects. By using means external to the vessel to rotate one of the levitating magnetic bearing or the superconducting element, the desired pumping or mixing action is provided.
As should be appreciated from reviewing the foregoing description, several advantages may possibly be provided through the use of the mixing or pumping system of the present invention, depending in part upon the particular application. Since the rotating magnetic bearing levitates in the fluid, there is no mechanical stirrer or mixing rod extending through any wall of the vessel, which means that the vessel can be completely sealed from the outside environment, if desired. This eliminates the need for a dynamic bearing or seal and the concomitant problems with leakage, sterility, and the like, which makes the present arrangement particularly well suited for use in pumping or mixing ultra-pure or hazardous fluids. Furthermore, exceptionally stable levitation of the magnetic bearing is provided by the minimal separation distance between the superconducting element and the magnetic bearing. Due to the thermal isolation and separation of the superconducting element from the cryostat wall, the system may even be used to pump or mix temperature sensitive fluids. In any case, contact-free, stable levitation reduces the incidence of frictional heating or unwanted shear stresses, both of which can have a significant deleterious effect on sensitive fluids, such as cell suspensions or the like.
In one possible embodiment, the magnetic bearing includes first and second spaced permanent magnets, which may be mounted at the opposite ends of a support shaft. The first magnet is placed in the fluid vessel closest to the outer wall of the cryostat such that it is levitated by the superconducting element. While those of skill in the art will understand that the polarity of the first permanent magnet is not critical for producing the desired levitation, it is preferred that it is disk-shaped and polarized in the vertical direction. This ensures that the magnetic field generated is substantially symmetrical and the desired stable levitation and free rotation relative to the vertical axis results.
The second permanent magnet forms a magnetic coupling with the motive device for rotating the magnetic bearing, which is preferably a drive magnet coupled to the rotating shaft of a motor. In applications where the stability of the magnetic bearing is particularly important, the drive magnet includes more than one magnet, and in the one embodiment has at least two sub-magnets that correspond to opposite polarity sub-magnets forming a part of the second permanent magnet. In addition to creating the desired magnetic coupling for transmitting the driving torque, these cooperating sub-magnets produce an attractive force that balances with the levitational force provided by the superconducting element to keep the bearing properly balanced in the vertical direction. The cooperating sub-magnet pairs also keep the levitating bearing axially aligned and prevent side-to-side movement without the need for active control. In combination, the magnetic couplings created by the sub-magnet pairs allow the bearing to rotate in an exceptionally stable fashion. This reduces the chances of the bearing inadvertently contacting between the bottom and side walls of the vessel, and eliminates the need for electromagnets, roller bearings, or like structures found in prior art pumps or mixers.
In an alternate version of the magnetic bearing, at least one, and preferably a plurality of chambers are provided for holding a gas or other substance that is lighter than the fluid or other substance surrounding the bearing. These chambers serve to assist in levitating the magnetic bearing in the fluid, while the pinning forces created by the superconducting element simultaneously assist in keeping the magnetic bearing properly positioned at the desired location in the vessel. The chamber or chambers thus effectively reduce the amount of levitation force that must be supplied by the superconducting element.
The superconducting element may be formed of melt-textured Yttrium-Barium Copper Oxide (YBCO), which is a well-known high temperature, or xe2x80x9cType II,xe2x80x9d superconducting material, formed into a relatively thin pellet. The thermal link between the superconducting element and the cooling source is created by an elongate rod formed of a material having desirable thermal transfer characteristics. Metals, such as copper, brass, aluminum, or the like, are particularly well-suited for this purpose, but the use of any other material having good thermal conductance/low thermal resistance is possible. The rod may be cylindrical in shape such that one end has a relatively large surface area that fully contacts and engages an entire face of the superconducting element to maximize thermal transfer. While one end of the rod supports the superconducting element in the chamber defined by the outer wall of the cryostat or other housing, which remains at room temperature, the opposite end is kept in thermal contact with the cooling source. The cooling source may take the form of a separate cooling chamber in the cryostat holding a cryogen ataur between 4.2 and 130 Kelvin, and most preferably liquid nitrogen at a temperature between approximately 65-80 Kelvin. Instead of liquid cryogens, the use of alternate means for cooling the rod is possible, such as providing a separate closed cycle refrigerator that is kept entirely outside of the cryostat or other housing for the superconducting element.
Since the magnetic bearing levitates without the need for a mixing rod or other form of driving shaft, it should be appreciated that the vessel containing the fluid may be completely sealed from the outside environment and used to mix, rather than pump, the fluid. By using such an arrangement, the potential for leakage or contamination during mixing is eliminated, as is the risk of exposing hazardous or biologically active fluids to the environment. Forming the sealed vessel and the magnetic bearing from disposable materials is also possible, such that both can simply be discarded after mixing is complete and the fluid is retrieved or recovered, if necessary. This advantageously avoids the need for clean up or sterilization of the vessel and bearing.
Also, since there is no need for a dynamic bearing or seal for any drive shaft penetrating through a wall of the vessel, the vertical center axis of rotation of the magnetic bearing can be easily offset from the vertical center axis of the vessel. The vessel can then be rotated in a direction counterclockwise to the rotation of the bearing mounted in such an offset position. By doing so, gentle, yet thorough mixing may be provided in an efficient manner.
It should also be appreciated that other alternatives to a sealed vessel are possible. Of course, the vessel may simply be open to the ambient environment, as may be desired during the mixing of some solutions or suspensions that require exposure to open air to achieve a desired result. Alternatively, the vessel may be substantially sealed with only an inlet and an outlet, such that the rotating magnetic bearing/impeller provides pumping action to move the fluid through the vessel. Manufacturing the open top or substantially sealed vessel of disposable materials is also possible, such that both the vessel and magnetic bearing can simply be discarded after use to avoid the need for clean up or sterilization. The vessel can also be a flexible bag or other non-rigid type of container, the dimensions of which are essentially defined by the volume of fluid held therein.
As should further be appreciated, the system described above is based on the use of a stationary superconducting element and a magnetic bearing that includes a levitation magnet and separate xe2x80x9cdrivenxe2x80x9d magnets. The driving force is applied to the driven magnets from adjacent the top of the vessel, while the levitation force is provided by the other, levitating magnet adjacent to the bottom of the vessel. While this system provides the several advantages described above, in many practical applications, it is advantageous to keep the top of the mixing vessel or pumping head substantially clear from obstructions. For instance, if the mixing vessel includes a number of different ports and connections on the top, such as a filling port, temperature sensor connector, pH sensor connector, or the like, driving the levitating magnetic bearing from the top may interfere with these structures, thus possibly making operation somewhat inconvenient. This is also true in the case where the levitating magnetic bearing is used in a pumping chamber or centrifugal pumping head, where it is often desirable to place the fluid inlet in the top or upper wall of the vessel.
Moreover, in case of accidental decoupling of the driving magnet with the driven magnet at the opposite side of the levitating magnetic bearing, the shaft may lose vertical stability and fall into contact with the bottom or sides of the container. If this occurs, it is impossible to recover the stable levitation without opening the container, if sealed, or otherwise disturbing the fluid. This, of course, can lead to deleterious contamination.
Yet another reason for providing an alternative to the top driven arrangement is that it eliminates the need for a fixed height vessel or container for holding the fluid. For example, in the case of where the vessel is in the form of a flexible bag, the vertical dimension of the bag often depends on the amount of fluid present, as well as the size and overall geometry of the bag itself. By magnetically driving a low-profile, levitating bearing or impeller in a stable, non-contact fashion from only the bottom of such a flexible plastic container, it could be of a reduced vertical dimension without compromising the degree of pumping or mixing action created.
Driving and levitating a magnetic bearing from the same side of the vessel also reduces the number of permanent magnets required. This is because the levitation magnets may simultaneously serve as the xe2x80x9cdrivenxe2x80x9d magnets. Eliminating the total number of magnets required not only reduces the materials cost, but also creates a bearing that is less complicated to manufacture.
Thus, another purpose of the present invention is to provide a magnetic bearing, and most preferably a low-profile, disk-shaped magnetic bearing or impeller (with or without blades, vanes, or the like) that is both levitated by a superconducting element and magnetically driven by means located outside of the vessel, and preferably on the same side of the vessel as the superconducting element. The magnetic bearing can thus be used for mixing or pumping fluids in a variety of vessels without regard to height, including flexible containers, such as bags or the like. Also, as described above, the magnetic bearing can be used along with a disposable plastic container (or with disposable impeller blades along with a disposable plastic pumping chamber or head).
To achieve this second goal, another version of a pumping or mixing system using a levitating magnetic bearing is disclosed. In this version, the thermally isolated superconducting element is contained within a wall defining a chamber that may be evacuated or insulated to create the desired thermal separation, as above, but instead of rotating the magnetic bearing including separate drive magnets, the motive device rotates both the wall and the superconducting element together. Accordingly, both the levitation and motive forces for the magnetic bearing are supplied by the same superconducting element (which actually can be formed of several component parts). To ensure proper rotation of the bearing, it includes at least two permanent magnets having different polarities that together create a non-symmetrical magnetic field with respect to the axis of rotation of the superconducting element. The bearing may also carry one or more blades or vanes to enhance the mixing or pumping action. In an alternate version of this embodiment, the cooling source may also be rotated along with the wall creating the chamber for thermally isolating the superconducting element (or may serve to couple the chamber to the motive device). In either case, the low-profile, magnet-carrying bearing may thus be efficiently and effectively levitated and rotated from the bottom of a vessel (or pumping chamber/head) resting on a stable support structure, while at all times remaining thermally separated and isolated from the cold superconducting element.
When using a vessel having a narrow opening, it may be difficult or impossible to insert the typical pancake or disk-shaped magnetic bearing in the fluid. Thus, an alternate version of a magnetic bearing, and one especially adapted for use in the pumping or mixing system of this second embodiment, is disclosed. The magnetic bearing is in the form of a low-profile rod. Each end of the rod carries a magnet. These magnets may serve as both the levitating and the driven magnets in the case where the bearing is positioned above a rotating superconducting element.
In another version, two of the low-profile rods, each carrying at least two magnets having identical polarities, are pinned together, preferably at their centers. The rods are thus capable of rotating relative to each other to form a low-profile magnetic bearing that can easily pass through a narrow opening in a vessel. Since the magnets at the end of each rod have the same polarity, they not only serve to levitate and drive the bearing, but also repel each other to keep the rods from aligning when rotating in the vessel. Instead of or in addition to pinning the rods together, it is also possible to fabricate one or both rods of a flexible material, and possibly a single integral piece of material. As a result of the flexibility, the bearing formed from the rigidly coupled rods can be deformed to pass through any narrow opening in a vessel or container.
Despite the preference for using the system of this second possible embodiment for pumping or mixing temperature sensitive fluids in view of the beneficial nature of the thermal separation, it should be appreciated that it is possible to use it for pumping or mixing non-temperature sensitive or cryogenic fluids as well. For example, the evacuated housing or like chamber surrounding the rotating superconducting element can be eliminated altogether, since there is no need to thermally separate it from the support structure for the vessel containing the magnetic bearing when a cold or non-temperature sensitive fluid is being pumped or mixed. Nevertheless, the desired stable, contact free levitation is still achieved.
In accordance with a third aspect of the present invention, the thermally isolated superconducting element provides the levitation, substantially as described above, while a separate motive device positioned adjacent to the superconducting element serves to rotate the magnetic bearing. In a most preferred version of this embodiment, the superconducting element is annular and positioned in a correspondingly shaped chamber defined by the outer wall of a cryostat or like device. This chamber may be evacuated or insulated to provide the desired thermal separation and isolation for the superconducting element. The wall also defines a bore or opening in the center of the chamber housing the superconducting element for receiving a portion of a motive device, such as a shaft carrying alternating polarity driving magnets at one end. The opposite end of the shaft is coupled to a motor also forming a part of the motive device. The magnetic bearing, in turn, carries a first xe2x80x9clevitatingxe2x80x9d magnet corresponding in shape to the superconducting element, as well as at least two alternating polarity xe2x80x9cdrivenxe2x80x9d magnets that couple with the corresponding driving magnets. This magnetic coupling with the driven shaft serves to provide the desired rotation for the levitating bearing, while the superconducting element simultaneously serves to levitate the bearing in the vessel.
To provide the necessary cooling, a thermal link connects the superconducting element with a separate cooling source, such as a container holding a suitable liquid cryogen or a closed-cycle refrigerator. Preferably, like the superconducting element, the rod and cooling source are each held in evacuated or insulated chambers to prevent any thermal transfer to or from the outside environment. In the case of evacuation, all three chambers are preferably in communication, but each may also be kept separate, such as by partitions, and individually evacuated or insulated. Thus, like the second embodiment, this system can also efficiently and effectively rotate a thermally isolated and separated magnetic bearing in a vessel containing a fluid to provide the desired pumping or mixing action. However, it should be appreciated that, like in the second embodiment, thermal separation is not a critical requirement, since the system of this embodiment could also be used to pump or mix non-temperature sensitive or cryogenic fluids as well.
In accordance with a fourth aspect of the invention, the vessel is in the form of a pipe containing a stationary or passing fluid. A correspondingly shaped superconducting element, which is preferably provided in two spaced component parts, surrounds the pipe. Each element is thermally separated and isolated from the outer surface of the pipe, such as by evacuating a chamber defined by a wall surrounding the element or filling it with insulation. A bearing positioned in the pipe carries levitating magnets corresponding in number to the components of the superconducting element and preferably positioned at each end of the bearing to ensure that a stable levitation force is achieved. As described above, the superconducting element may be thermally linked to a separate cooling source, such as a liquid nitrogen container, refrigerator, or the like. This link provides the necessary cooling such that the superconducting element causes the magnetic bearing to levitate in the pipe in a stable and non-contact fashion.
To rotate the bearing, it may also carry a plurality of driven magnets that correspond to driving magnets positioned externally to the vessel and rotated by a motive device. Alternatively, a winding may be provided around the vessel and supplied with an electrical current to create an electric field that induces rotation in the driven magnets carried on or attached to the bearing. In either case, a levitating, rotating magnetic bearing is provided for xe2x80x9cinlinexe2x80x9d use in a pipe or other narrow, elongated vessel.
In an alternate xe2x80x9cinlinexe2x80x9d embodiment, the cryostat or other wall defining a chamber for housing the superconducting element is positioned in the vessel, such that the superconducting element aligns with and corresponds to a levitation magnet in the bearing, while separate, room-temperature driving magnets forming a part of a motive device correspond to and align with opposite polarity driven magnets in the bearing to form a magnetic coupling. The chamber is preferably evacuated or insulated to thermally isolate the superconducting element from the bearing and the surrounding fluid. A separate cooling source is also provided to supply the necessary cooling to the superconducting element to induce levitation in the bearing. The superconducting element and surrounding chamber may both be annular, as in the third embodiment. The inner wall creating this annular chamber also defines a bore for receiving the end of a driven shaft carrying the driving magnets for coupling with the adjacent driven magnets on the bearing. The bearing may also carry one or more blades or vanes to enhance the pumping or mixing action.
In accordance with a fifth aspect of the present invention, an assembly for use in containing a fluid undergoing pumping or mixing is provided. The assembly comprises a vessel formed of a flexible disposable material capable of holding a fluid and a magnetic bearing positioned in the vessel. Thus, when used in conjunction with a pumping or mixing system wherein the magnetic bearing is levitated in the vessel by an adjacent superconducting element, both the vessel and the bearing can be disposed of when the pumping or mixing operation is complete and the fluid is recovered. While not an exhaustive list, the vessel can be selected from the group of an open-top container, a pipe, a container having an inlet for receiving a flow of fluid and an outlet for expelling a flow of fluid, a sealed container, or a flexible bag. An attachment or cover containing a coupler comprised of a ferromagnetic material or the like may also be provided to keep the bearing in the proper position relative to the bag or vessel, such as during shipping or the like.
Ensuring that the magnetic bearing used in each system is both the proper one for that particular system and is sized appropriately may also be important. To do so, and in accordance with a sixth aspect of the invention, it is possible to provide a transmitter in one of the magnetic bearing or the vessel for generating a signal that is received by a receiver positioned elsewhere in the system (or vice versa), such as one positioned adjacent to the superconducting element. A controller for the system, such as a computer, can then be used to maintain the system in a non-operational, or xe2x80x9clock-out,xe2x80x9d condition until such time as the appropriate signal is received.
In accordance with a seventh aspect of the invention, a kit is also provided to assist in the set-up of any of the systems previously described. Specifically, it is necessary during field cooling to cool the superconducting element to below its transition temperature in the presence of a magnetic field in order to induce levitation in a permanent magnet producing the same magnetic field. This cooling process causes the superconducting element to xe2x80x9crememberxe2x80x9d the magnetic field, and thus produce the desired stable and reliable levitation each time a similar field is present. While it is possible to use the magnetic bearing to produce the magnetic field during field cooling, oftentimes the bearing will be pre-sealed in the vessel or container. This makes it difficult, if not impossible, to ensure that the magnet is properly aligned and spaced from the superconducting element during field cooling.
To overcome this problem, the kit of the present invention comprises at least one charging magnet having a size, shape, and magnetic field distribution identical to the levitation magnet contained in the particular bearing slated for use in one of the pumping or mixing systems previously described. The charging magnet is placed adjacent to the superconducting element during cooling, such as on the upper surface of the cryostat or other chamber surrounding the superconducting element (or a stable support structure for the bearing). Once cooling below the transition temperature is complete, the charging magnet may be removed and replaced with the vessel containing the corresponding magnetic bearing.
The kit or charging magnet may also comprise a spacer. This spacer allows the charging magnet to simulate the spacing of the magnetic bearing above the superconducting element to ensure that the desired levitation height is achieved once the vessel containing the actual bearing to be levitated is in position. The spacer is fabricated of a non-magnetic material to avoid interfering with the charging process. By also providing a variety of different sizes, shapes, and configurations of charging magnets (e.g., annular magnets), it is possible to easily perform field cooling for any size or shape of levitation magnet in the corresponding magnetic bearing.
During field cooling, and regardless of whether the magnetic bearing or a separate charging magnet is used to produce the charging magnetic field, it is possible to induce an undesired magnetic state in the superconducting element, such as if the position of the bearing or charging magnet is not correct during cooling. Since improper charging may prevent the magnetic bearing from levitating in a stable fashion, xe2x80x9crechargingxe2x80x9d the superconducting element may be required.
In accordance with an eighth aspect of the present invention, a heater may be provided adjacent to the superconducting element for use in facilitating recharging. More specifically, by activating this heater, the superconducting element may be quickly brought up from the transition temperature for recharging. Once the position of the bearing or charging magnet is adjusted or corrected, the heater may be turned off and the superconducting element once again allowed to cool to the transition temperature in the presence of the desired magnetic field. Of course, this operation may be repeated as necessary until the desired stable levitation is achieved.
In many of the above-described embodiments, the pumping or mixing action is essentially localized in nature, since the bearing is rotated on a fixed axis relative to the vessel. This may be undesirable in some situations, such as where the vessel is relatively large, as compared to the magnetic bearing. Thus, in accordance with a ninth aspect of the invention, the particular system used to supply the pumping or mixing action may be provided with a motive device for physically moving the superconducting element (which may also be simultaneously rotated). Moving the superconducting element relative to the vessel will cause the levitating magnetic bearing to follow a similar path.
In accordance with a tenth aspect of the present invention, a method of levitating and rotating a magnetic bearing, such as for pumping or mixing a fluid in a vessel, is disclosed. The method includes the steps of placing a magnetic bearing in the vessel. Levitation is induced in the magnetic bearing by a superconducting element, which may be positioned in an insulated or evacuated chamber defined by the outer wall of a cryostat or other housing. If present, the chamber serves to thermally isolate and separate the vessel, fluid, and magnetic bearing from the superconducting element, which is thermally linked to a separate cooling source. Upon rotating one of the levitating magnetic bearing or the superconducting element in the vessel, the desired mixing or pumping action is provided. As described above, the magnetic bearing and vessel may also be formed of disposable materials and discarded once mixing is complete and the fluid is recovered. Other methods are also disclosed for accomplishing the goals of the other embodiments previously described.